1. Field of the Invention
The present invention relates to an improvement of a dual gate GaAs FET.
2. Description of the Prior Art
Recently, the dual gate GaAs FETs which have a superior high frequency characteristic as well as a high gain are being developed.
The main feature of the dual gate structure FET is a superior gain control characteristics. That is, by impressing a D.C. bias voltage on a second gate, a gain control with small input and output detunings is easily obtainable. In case, the dual gate structure GaAs FET is used in a tuner or a front end stage of a television set, it is necessary to design its drain to source saturation current I.sub.DSS to be smaller than 40 mA (which corresponds to the voltage V.sub.G2SC second-gate and the source to cut off the drain current of 3.0 V). Such dual gate structure GaAs FET having the I.sub.DSS under 40 mA has had a poor cross modulation characteristic, especially at large gain reduction range, thereby forming a major cause of hindering wide actual use of such FET in the home use in television tuners.
In general, the distortion is dependent on the linearity of a curve between voltage V.sub.G1S between the second gate and the source and mutual conductance Gm (which is defined by Gm=dI.sub.DS /dV.sub.G1S, where I.sub.DS is a drain to source current), and in amplifiers, the distortion mainly consists of a third distortion which is induced by a secondary term of the curve. By expressing the drain current I.sub.D as follows: EQU I.sub.DS =I.sub.DO +.alpha.V.sub.G1S +.beta.V.sub.G1S.sup.2 +.gamma.V.sub.G1S.sup.-3 ( 1),
where I.sub.DO, .alpha., .beta. and .gamma. are constants, and the amplitude Au of undesired signal by EQU A.sub.u =V.sub.u (1+m.sub.u cos pt) (2),
where m.sub.u is a constant, then, intensity of the distortion m.sub.k is expressed by: EQU m.sub.k =3(.gamma./.alpha.)V.sub.u.sup.2 m.sub.u ( 3).
In a high frequency range, the linearity of the Gm can be regarded substantially dependent on the linearity of the S parameter .vertline.S.sub.21 .vertline. with respect to the gate voltage V.sub.G1S. FIG. 1 shows characteristic curves of relations between the S parameter .vertline.S.sub.21 .vertline. at 1 GHz and gate bias voltage V.sub.G1S first gate to source voltage) of a conventional Schottky barrier dual gate GaAs FET for ordinary communication use where I.sub.DSS is 50 to 70 mA (corresponding to V.sub.G2SC (second gate to source voltage to cut off drain current) of 3 to 5 V). In the conventional Schottky barrier dual gate GaAs FET, both the lengths of the first gate and the second gate are designed about 1 .mu.m. FIG. 2 shows cross-modulation characteristic of the abovementioned dual gate GaAs FET, wherein the ordinate is graduated with such levels of undesired signal of 30% modulation that which induces a 1% distortion to a desired signal and the abscissa is graduated by gain reduction (in dB). The curve of FIG. 2 is obtained by changing the first gate bias V.sub.G1S and the second gate bias V.sub.G2S along an oblique broken line in FIG. 1, that is by changing
V.sub.G1S . . . from -2 V to -0.5 and PA1 V.sub.G2S . . . from +2 V to pinch off voltage (-3 to -5 V), respectively, where the undesired signal frequency or the interference signal frequency fu is 762 MHz and desired signal frequency fd is 750 MHz. In an actual amplifier, the curve of FIG. 2 should be in the area defined as above the dotted oblique broken line therein. The curve of FIG. 2 has two sags, one at the gain reduction of about -5 dB and the other at the gain reduction of about -20 dB, showing poor cross-modulations around these areas, which correspond to the parts of poor linearity of the curves in FIG. 1. Though there is some sags in the characteristic curve of FIG. 2, the curves being above the broken line fulfills actual demand for the cross modulation. However, when the drain current I.sub.DSS is set under 40 mA in order to fulfill the demand for home-use appliances, the distortion characteristics become worse, thereby hindering wide usage of the FET in actual circuits. FIG. 3 shows the cross-modulation characteristic of another dual gate GaAs FET, wherein I.sub.DSS is set to such a small value as 20 mA. As shown by the curve of FIG. 3, such dual gate GaAs FET of small I.sub.DSS has a poor cross-modulation characteristic around the gain reduction of -30 dB. The inventors confirmed through experiments that the smaller the I.sub.DSS is set, the poorer the distortion characteristic becomes.